1. Field of the Invention
The present invention relates generally to deep sea drilling equipment and in particular to the construction and operation of a deep sea offshore drilling and development platform having dual work stations for simultaneously conducting oilfield operations from each of the work stations. More particularly, the present invention relates to systems and apparatae required to permit drilling and/or production operations simultaneously from two adjacent positions on a floating platform while preventing operations being conducted by one of the work stations from interfering with the operations being conducted by the other work station. Most notably, the invention includes means for determining the relative position between two marine risers depending from two respective work stations based on environmental and other parameters. Such means also provides the capability to calculate the effect of changes in system parameters induced specifically to control the relative position of the two conductor pipes (marine risers).
2. Description of the Prior Art
Search for offshore deposits of crude oil and natural gas is continually being extended into deeper and deeper waters beyond the continental shelf. Where possible, one of the preferred techniques for performing the operations necessary for the production of hydrocarbons from offshore reservoirs is to erect a structure or drilling platform which is in some fashion rigidly secured to the sea floor. Such a technique may comprise any of a variety of structures including jackup rigs, tension leg platforms, free standing or guyed towers. A notable advantage of such structures is their rigid nature which significantly simplifies subsea operations during conditions which exert lateral or vertical forces on the structure. The rigid character of such structures limits their movement to less than four degrees of freedom (0 for a rigid tower and up to 3 for a tension leg platform). It has, therefore, been found possible to operate a pair of drilling derricks on such rigid, bottom founded structures. With such rigid structures operation of two conductor pipes simultaneously has not caused significant problems to arise due to the active guidance available with these systems. U.S. Pat. Nos. 2,973,046 and 4,170,266 are illustrative platforms which are supported in a rigid or semi rigid fashion from the sea floor. There is, however, a limit to the water depths in which rigid, bottom founded drilling platforms can be effectively, safely and economically operated Where the sea depth exceeds this limit, floating platforms such as drilling ships or semi-submersible platforms have found application. According to conventional procedures, a floating drilling vessel is dynamically moored above a well site on the ocean floor. Dynamic mooring, as opposed to rigid, bottom founded support permits the floating platform to dynamically move with up to six degrees of freedom, under prevailing forces such as wave action, tidal action, sea currents and wind conditions. This dynamic mooring somewhat complicates the equipment and procedures employed in drilling a well in the sea floor. In this process, a wellhead is first manipulated into position and installed on the sea floor. Then a blowout preventer (BOP) stack is lowered from the floating rig and mounted on top of the casing of the wellhead as a means for controlling the pressures which may arise at the surface opening of the well. The installation of such equipment is conventionally performed by lowering the equipment from the rig on the end of marine risers. A drill string is next extended from the floating vessel to the bottom of the well through the wellhead equipment on the ocean floor. The drill string is enclosed within the riser pipe which has been attached to the wellhead equipment and which is supported under tension from the floating drilling vessel to prevent its collapse. Drilling mud is circulated through the drill string and is returned through the riser annulus which surrounds the drill pipe. As can be easily imagined, the marine risers required for drilling and production operations in very deep water become quite heavy and unwieldily. Unfortunately, the movement of a floating drilling vessel under the influence of weather, tide and current conditions greatly increases the difficulty of managing the riser as contrasted to the situation of a rigid, bottom founded platforms since movement of the vessel excites dynamic motions in the riser systems.
Drilling vessels and other apparatus employed in the drilling of oil wells offshore are generally large and very expensive and their daily operation involves rates exceeding many thousands of dollars a day, a cost which constitutes a major portion of the overall well cost. Thus, it is very important that the drilling operations of such a vessel be performed with as little interruption as possible. In the past, standard practice has been to sequentially prepare a plurality of wells from a floating vessel with a single drilling platform. Once drilling is complete and the wells have been placed in condition for operation, the floating drilling vessel is typically removed and replaced by a floating production facility. This is attendant with numerous disadvantages including the increased cost due to the removal and replacement of the floating drilling vessel with a floating production platform, the need for preparing and maintaining redundant vessels (one for drilling and the other for production), and the loss of considerable amounts of time during which the floating drilling vessel is replaced with the floating production facility. Additional time is lost and expense incurred in the not uncommon occasion in which a drilling rig is once again required at the well site. For example, producing wells are known to develop waxy buildups or other conditions which reduce or jeopardize the fluid flow from the formation into the well and which require a workover operation that can be conducted only from a drilling rig. As a result, the production facility may have to be removed and replaced with a floating drilling vessel which contains the drilling rig in order to perform the workover operations.
As previously suggested one approach that has availed itself with bottom, vertically upwardly supported fixed platforms has been the provision of two, side by side drilling rigs on the same platform. Such an arrangement enables a more efficient operation due to the ability to drill approximately twice as many wells in the same amount of time with fewer people and with shared equipment. This approach has not previously been thought suitable for the floating vessel due to the tendency of floating vessels to move about under the prevailing weather, sea current and tidal conditions which, as previously mentioned, tends to produce motion in the marine risers. Clearly, if certain limits of tension, or deflection angle are exceeded, a marine riser can be damaged. Damage may also occur if two risers were to forcibly come into uncontrolled contact with one another or if equipment being lowered by one riser were to collide with the other riser. Drilling risers, while quite stiff over short distances, are quite flexible over the extended distances which they must traverse in the deep water offshore environment. Not only are these risers subjected to sea currents (often of different magnitudes and directions at different depths and times), but they are also subjected to a condition in which the lower end is pinned to the ocean floor at a stationary spot while the upper end must follow the motions of the floating platform.
Numerous attempts have been made in the past to deal with problems which arise in the design and management of marine risers for deep, offshore oil well drilling and production. For example, U.S. Pat. Nos. 3,983,706 and 3,817,325 disclose means for providing lateral support and guidance to marine risers in order to limit their lateral deflection due to currents or platform movement U.S Pat. Nos. 3,601,187; 4,576,516; and 4,188,156 describe flexible joints and flexible riser sections for the purpose of accommodating unavoidable deflection of such risers. U.S. Pat. Nos. 3,313,345 and 4,351,261 disclose apparatus and techniques for preventing the violent collision, of a riser with the floating platform if the riser were to be separated from the wellhead equipment in a planned or emergency disconnect situation. U.S. Pat. Nos. 3,434,550 and 3,999,617 are directed to methods and apparatus for lightening the riser-mud combination to reduce the compressive and tensile forces placed on the riser. And U.S. Pat. Nos. 4,142,584 and 4,198,179 show the conventional approach of ganging multiple risers together as a means of avoiding riser-riser interference.
Many of the problems associated with the operation of a floating drilling vessel having dual work stations, each of which may be outfitted with either a drilling rig or a production facility, are substantially reduced or eliminated by use of the present invention which provides for adequate control of a pair of parallel risers required by the operations conducted on a floating drilling vessel having side by side work stations.